Tracked machines, e.g., track-type tractors, tracked loaders, and the like, generally employ one of two methods for steering. Each method offers unique advantages and disadvantages. Differential steering, for example, operates by applying differential power to each of a left and a right track on the tracked machine. A desired turn is accomplished by applying additional power to one of the tracks to increase the speed of the first track relative to the second track. Differential steering offers an advantage of highly accurate and controlled steering. However, differential steering is costly, the cost increasing dramatically as the machines become larger.
A second method, clutch/brake steering, costs less to implement than differential steering. However, clutch/brake steering does not provide the accuracy and control of differential steering. Clutch/brake steering functions by reducing speed at one of the tracks relative to the other track by engaging and disengaging either a clutch or brake or both for one track.
It is common for tracked machines to employ lower cost clutch/brake steering rather than differential steering, and sacrifice some level of accuracy and control. As a result, attempts have been made to compensate for some of the control disadvantages inherent with clutch/brake steering systems. For example, it is normally desired to disengage a clutch prior to engaging a brake to reduce sudden jerky movements of the tracked machine. This results in a deadband between clutch and brake response. This method works fairly well on level ground; however, if the tracked machine is on a slope, the weight of the machine causes loss of steering control during the deadband period.
In U.S. Pat. No. 5,325,933, Matsushita compensates for this problem by sensing the presence of a slope and responsively reducing the deadband between clutch engagement and brake disengagement. However, Matsushita does not account for a number of other variables, such as the initial velocity of the tracked machine, the desired turn radius, and the physical capability of the tracked machine to successfully negotiate a desired turn under various circumstances.
The use of clutch/brake steering systems becomes more complex as tracked machines are being developed which operate autonomously, semi-autonomously, or by remote control. Parameters of the machine which can be compensated for by a skilled operator, such as weight, center of gravity, velocity, desired turn radius, and various torques and forces being applied during movement of the machine, must be accounted for during a clutch/brake steering maneuver to maintain control of the machine. The above parameters must be evaluated to determine the manner in which the clutch/brake steering system operates.
The Matsushita patent described above is designed to enhance operator control of a tracked machine on a slope. However, the other parameters needed to operate the tracked machine in an autonomous or semi-autonomous manner are not compensated for by Matsushita. The Matsushita patent provides a means for clutch/brake steering to function more accurately when a machine is on a slope, but does not provide a means for a tracked machine to use clutch/brake steering autonomously.
In U.S. Pat. No. 4,702,358, Mueller et al. uses clutch/brake steering by sensing left and right track velocities, determining the difference between the left and right sensed velocities, comparing the sensed velocity differences to desired left and right track velocity differences, and responsively steering the tracked machine. However, for accuracy and control that approaches the level supplied by a differential steer system, in particular for a tracked machine operating autonomously, an improved clutch/brake steering control system is needed.
The present invention is directed to overcoming one or more of the problems as set forth above.